Mask set for use in phase shift photolithography technique which is suitable to form random patterns, and method of exposure process using the same

ABSTRACT

A method of forming a photoresist pattern by a photolithography technique is composed of:  
     providing a photoresist layer;  
     exposing the photoresist layer to a first pattern-defining light using a first mask; and  
     exposing the photoresist layer to a second pattern-defining light using a second mask. The first mask includes a shielding region shielding the first pattern-defining light. The second mask includes a phase-shifting region having a phase shifter edge and a non-phase-shifting region adjacent to the phase-shifting region on the phase shifter edge. A first light portion of the second pattern-defining light passes through the phase-shifting region. A second light portion of the second pattern-defining light passes through the non-phase-shifting region. A first phase of the first light portion differs from a second phase of the second light portion. The first and second masks are aligned such that the phase shifter edge overlaps on the shielding region.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to an exposure method for use inphotolithography and a mask for use in photolithography. Moreparticularly, the invention relates to a phase-shift mask and anexposure method using the phase-shift mask.

[0003] 2. Description of the Related Art

[0004] To form patterns of semiconductor elements, a photolithographytechnique is commonly employed. A pattern of a mask is transferred to aphotosensitive resin layer provided on a semiconductor substrate by thephotolithography technique. The photosensitive resin is also known as“resist.” A resist is classified into two type, i.e., negative type andpositive type. The negative-type resist is of the type; any part ofwhich that has been exposed to the light applied through a mask willremain on the semiconductor substrate. The positive-type resist is ofthe type; any part of which that has been exposed to the light appliedthrough a mask will be removed from the semiconductor substrate.

[0005] In recent years, it has been demanded that an image be formed ona resist layer in higher resolution to provide fine patterns ofsemiconductor elements. Fine semiconductor element patterns increaseintegration density of a semiconductor integrated circuit.

[0006] To enhance the resolution of an image formed on a resist, a phaseshift exposure method is proposed in 1982. In the phase shift exposuremethod, the phase difference between light beams applied is utilized toimprove the resolution of the image focused on a resist layer. Theprinciple of the phase shift exposure will be described, with referenceto FIGS. 1A to 1D and FIGS. 2A to 2D.

[0007] In the ordinary exposure, the light applied perpendicularly to amask 106 passes through the transparent regions 150 and 151 of the maskas illustrated in FIG. 1A. Chromium mask patterns 121 are provided onthe mask 106. The mask 106 has transparent regions 150 and 151. Thelight beams passing through the transparent regions 150 and 151 have thesame phase. The light beams emanate from the transparent regions 150 and151 and pass through the projection lens of a reducing projectionexposure apparatus. The two beams are then focused on the surface of aresist layer, which is on an image-forming surface.

[0008] The distance between the transparent regions 150 and 151 cannotbe reduced to an infinitesimal value, for the following reason. If thedistance is extremely short, the two beams passing the regions 150 and151 overlaps at the image-forming surface as indicated by the brokenlines in FIG. 1C. The light beams, which have a same phase, intensifyeach other at the image-forming surface. As a result, thelight-intensity distribution on the surface of the resist has one peakas illustrated in solid line in FIG. 1D. Consequently, the chromium maskpatterns 121 are not correctly transferred to the resist layer. Thus,the interval between the transparent regions 150 and 151 cannot bedecreased over a certain limit. The limit R of resolution for any imageformed on a resist is given as follows:

R=K ₁ ×λ/NA  (1)

[0009] where K₁ is the constant that depends on the properties of thephotosensitive resin, λ is the wavelength of the light applied to themask 106, and NA is the numerical aperture of the projection lens thatis incorporated in the reducing projection exposure apparatus. Here, thelimit R is known as “Reyleigh resolution”.

[0010] In the phase shift exposure, light is applied to a resist layerthrough a phase shift mask 107 as is illustrated in FIG. 2A. The phaseshift mask 107 has transparent regions 152 and 153. The region 153 isprovided with a phase shifter 120, while the region 152 has no phaseshifters. The light beam passing through the transparent region 153 isdelayed as it passes through the phase shifter 120. Hence, the lightbeam passing through the transparent region 153 differs in phase fromthe light beam passing through the transparent region 152. The thicknessD that the phase shifter should have to impart a phase difference of180° to the light beams is given as follows:

D=λ/{2×(n−1)}  (2)

[0011] where λ is the wavelength of the light applied to the phase shiftmask 107, and n is the refractive index of the phase shifter 120. If thetwo light beams emanating from the transparent region 152 and thetransparent region 153, respectively, have a phase difference of 180°,their parts overlapping at the image-forming surface will cancel outeach other. As a result, as shown in FIG. 2C, the intensity of light isnil at one part of the surface of the resist layer. It follows that thelight-intensity distribution on the resist has two peaks as shown inFIG. 2D. The chromium patterns 121 can therefore be transferred to theresist with high accuracy. Thus, the use of the phase shift mask 107 canenhance the resolution of an image focused on the surface of a resist.

[0012] Also, the phase shift exposure technique can increase the depthof focus (DOF). The term “depth of focus” means the range of distanceover which the focus may be displaced without causing troubles. Thereason is discussed comparing the ordinary exposure technique and thephase exposure technique in the following.

[0013] In the ordinary exposure using no phase shifters, the more theimage-forming surface deviates from the focal plane, the more the twobeam emanating from the transparent regions 150 and 151 overlap eachother at the image-forming surface. This means that the resolution willsharply decrease if the image-forming surface of the resist deviatesfrom the focal plane.

[0014] In the phase shift exposure, the two adjacent beams emanatingfrom the transparent regions 152 and 153 have a phase difference of180°. Their overlapping parts cancel out each other at the image-formingsurface of the resist layer. The intensity of light is therefore zero atone part of the image-forming surface. Hence, even if defocusing occurs,that is, even if the focus deviates from the image-forming surface, thedimensional precision of the pattern, transferred to the resist, will behardly influenced. Thus, the depth of focus can be increased in thephase shift exposure.

[0015] The phase shift exposure technique, however, cannot successfullyapply to two-dimensional random patterns. The layout pattern of asemiconductor integrated circuit includes regular patterns and randompatterns. Each regular pattern extends in one direction only, whereaseach random pattern randomly extends first in one direction, and then inanother direction. Here, examples of regular patterns are the bit linesand word lines of a DRAM (Dynamic Random Access Memory). Examples ofrandom patterns are the wires of logic circuits. The phase shift masksare designed in accordance with the basic rule that a phase differenceof 180° is imparted to two beams that have passed through two adjacenttransparent regions. This basic rule can be easily applied to theregular patterns, but not to two-dimensional random patterns.

[0016]FIG. 3A is a plan view of a phase shift mask 108 that may be usedto form two-dimensional random patterns by means of the conventionalphase shift exposure. The phase-shift mask 108 is designed to transfer apattern on a positive-type resist. The mask 108 has a shield region 111,a transparent region 113 and a transparent region 114. The shield region111 is identical in shape to the pattern that is to be transferred to aresist. Phase shifters 120 are provided on the transparent region 113.The beam passing through the transparent region 113 is out of phase withrespect to the beam passing through the transparent region 114. In otherwords, the phase of the beam differs by 180° from that of the beampassing through the transparent region 114. FIG. 3C is a sectional viewof the Levenson-type mask 108, taken along line C-C in FIG. 3A. As FIG.3C shows, the mask 108 is composed of a glass substrate 122. A chromiumfilm 121 is provided on the shield region 111, and a phase shifter 120is provided on the transparent region 113. No phase shifters areprovided on the transparent region 114.

[0017] Light is applied to the positive-type resist through thephase-shift mask 108. Light-exposed parts of the positive-type resistare developed and resist patterns 117 are formed as shown in FIG. 3B.The shield region 111 shields the part 115 of the positive-type resistfrom the light. Thus, the part 115 of the resist, which opposes theshield region 111, is developed as is intended.

[0018] In addition, the part 116 of the resist layer, which opposes theboundaries between the transparent regions 113 and 114, is developed.The beams passing through the transparent region 113 (having a phaseshifter) and the transparent region 114 (having no phase shifters) havethe opposite phases. The intensity of light is therefore almost nil atthe boundary between the transparent regions 113 and 114. The part 116of the resist, which opposes the boundary between the transparentregions 113 and 114, is also developed. That is, not only the part 115that should be developed, but also the parts 116 which should not bedeveloped is developed.

[0019] With the conventional phase shift exposure it is difficult toprevent the part 116 of the resist layer from being developed. Hence,the conventional phase-shift mask 108 cannot be used to transfertwo-dimensional random patterns to a positive-type resist.

[0020] It will be described now how two-dimensional random patterns aretransferred to a negative-type resist by means of the conventional phaseshift exposure technique. FIG. 4 is a plan view of a phase-shift mask109 that is used to transfer two-dimensional random patterns to anegative-type resist. As shown in FIG. 4, the phase-shift mask 109 has ashield region 111′, a transparent region 113′ and a transparent region114′. A phase shifter is provided on the shield region 111′. Thetransparent region 113 has phase shifters while the transparent region114′ has no phase shifters. The phase shifter imparts a phase differentof 180° to the beam that has passed through the transparent region 113′,with respect to the beam that has passed through the transparent region114′.

[0021] The transparent region 113′ is an auxiliary pattern for enhancingthe resolution of the negative-type resist pattern that will be formedat a position corresponding to the transparent region 114′. Nonetheless,the transparent region 113′ is required to have a width equal to or lessthan the value equivalent to the resolution limit. It is difficult toform transparent regions having such a small width at high reliability.Hence, the conventional phase shift exposure technique cannot processresists to form two-dimensional random patterns on negative-typeresists.

[0022] As described above, the conventional phase shift exposure cannotbe applied to transfer two-dimensional random patterns on positive-typeresists or negative-type resists.

[0023] It is desired that images be formed on resists at resolution highenough to form two-dimensional random patterns.

[0024] It is also desired that two-dimensional random patterns of highprecision be transferred to resists by means of phase shift exposuretechnique.

SUMMARY OF THE INVENTION

[0025] An object of the present invention is to form high-resolutionimages on resists in the process of forming two-dimensional randompatterns.

[0026] Another object of the invention is to transfer two-dimensionalrandom patterns of high precision to resists by means of phase shiftexposure technique.

[0027] In order to achieve an aspect of the present invention, a methodof forming a photoresist pattern by a photolithography technique iscomposed of:

[0028] providing a photoresist layer;

[0029] exposing the photoresist layer to a first pattern-defining lightusing a first mask; and

[0030] exposing the photoresist layer to a second pattern-defining lightusing a second mask. The first mask includes a shielding regionshielding the first pattern-defining light. The second mask includes aphase-shifting region having a phase shifter edge and anon-phase-shifting region adjacent to the phase-shifting region on thephase shifter edge. A first light portion of the second pattern-defininglight passes through the phase-shifting region. A second light portionof the second pattern-defining light passes through thenon-phase-shifting region. A first phase of the first light portiondiffers from a second phase of the second light portion. The first andsecond masks are aligned such that the phase shifter edge overlaps onthe shielding region.

[0031] The shield region may include a line resist shielding portion toform a line resist pattern extending to a first direction. The lineshielding portion has a centerline extending to the direction. In thiscase, it is desirable that the phase shifter edge substantially overlapson the centerline when the first and second masks are aligned.

[0032] The phase shifter edge may be composed of first and second phaseshifter edges parallel to each other and extending to the firstdirection. In this case, a distance between the first and second phaseshifter edges is desirably larger than a width of the line shieldingportion in a second direction perpendicular to the first direction.

[0033] The phase-shifting region may be provided with a phase-shifterlayer. In this case, a thickness of the phase-shifter layer is desirablydetermined such that a phase difference between the first phase and thesecond phase ranges from 175 to 185°.

[0034] Also, the first pattern-defining light has a first intensity andthe second pattern-defining light has a second intensity. In this case,the second intensity is desirably larger than the first intensity.

[0035] In order to achieve another aspect of the present invention, Amask set is composed of a first mask and second mask. The first maskincludes a shielding region shielding a pattern-defining light exposedto the first mask. The second mask includes a phase-shifting regionhaving a phase shifter edge and a non-phase-shifting region adjacent tothe phase-shifting region in the phase shifter edge. A first phase of afirst light passing through the phase-shifting region differs from asecond phase of a second light passing through the non-phase-shiftingregion. The phase shifter edge overlaps on the shield region when thefirst and second masks are aligned.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036]FIGS. 1A to 1D are diagrams explaining a conventional exposuretechnique;

[0037]FIGS. 2A to 2D are diagrams for explaining a conventional phaseshift exposure technique;

[0038]FIG. 3A is a plan view of a conventional Levenson-type mask thatmay be used to form two-dimensional random patterns;

[0039]FIG. 3B is a diagram showing parts of a positive-type resist whichare developed after exposed to light applied through the mask shown inFIG. 3A;

[0040]FIG. 3C is a sectional view of the conventional phase-shift mask108;

[0041]FIG. 4 is a plan view of another Levenson-type mask 109 that maybe used to transfer two-dimensional random patterns;

[0042]FIG. 5A is a plan view of a chromium mask 1(i.e., a photo-mask)used in the first exposure step of the exposure method according to theinvention;

[0043]FIG. 5B is a sectional view of the chromium mask 1;

[0044]FIG. 5C is a plan view of a chromium-less phase shift mask 2 usedin the second exposure step of the exposure method according to theinvention;

[0045]FIG. 5D is a sectional view of the chromium-less phase shift mask2;

[0046]FIG. 6A shows a layout of patterns obtained when the masks 1 and 2are laid one upon the other;

[0047]FIG. 6B is a sectional view of the chromium mask 1;

[0048]FIG. 6C is a sectional view of the chromium-less phase shift mask2;

[0049]FIG. 6D represents the intensity distribution of a laser beam 54applied to a positive-type resist 56 in the first exposure step;

[0050]FIG. 6E depicts the intensity distribution of a laser beam 58applied to a positive-type resist 56 in the second exposure step;

[0051]FIG. 6F represents half the total distribution of the light beamsapplied to the positive-type resist 56 in the first and second exposuresteps;

[0052]FIG. 7 shows the intensity distribution of light applied to aresist in the exposure method of the invention and the intensitydistribution of light applied to a resist in the conventional exposuremethod;

[0053]FIGS. 8A to 8F are diagrams showing the defocus-dependency of theintensity distribution of light applied to a resist, which is observedin the exposure method according to the invention;

[0054]FIGS. 9A to 9F are diagrams showing the defocus-dependency of theintensity distribution of light applied to a resist, which is observedin the conventional exposure method;

[0055]FIG. 10 is a plan view depicting another type of a chromium maskand another type of a chromium-less phase shift mask;

[0056]FIG. 11 is a sectional view of a chromium-less phase shift mask 2′designed for printing patterns on substrates; and

[0057]FIG. 12 shows the reducing projection exposure apparatus that isused to perform the exposure method according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0058] An exposure method according to an embodiment of the presentinvention will be described below, with reference to the accompanyingdrawings.

[0059] The exposure method according to the present embodiment includesthe first exposure step and the second exposure step. The first exposurestep is performed, using a chromium mask 1 shown in FIG. 5A. The secondexposure step is performed, using a chromium-less phase shift mask 2shown in FIG. 5B.

[0060] As shown in FIG. 5A, the chromium mask 1 has a shield region 11and a first transparent region 12. The shield region 11 defines atwo-dimensional random pattern. The term “two-dimensional randompattern” means a pattern composed of lines irregularly arranged, notcomposed of lines that extend in vertical direction or horizontaldirection, or both. In other words, a two-dimensional random pattern isone in which the interval between the constituent lines and/or thelength of each constituent line is not regular in the vertical directionand/or the horizontal direction. The shield region 11 defining atwo-dimensional random pattern has vertical lines and horizontal lines,each having a centerline 10. The shape of the shield 11 is determinedbased on the design values described in the layout data generated bymeans of CAD (Computer Aided Design).

[0061] As shown in FIG. 5B, the chromium mask 1 includes a transparentsubstrate 22. A shield film 21 is provided on a part of the transparentsubstrate 22 that corresponds to the shield region 11. The shield film21 reflects the light applied to the chromium mask 1 in the exposureprocess, not allowing the light to pass through the chromium mask 1. Noshield film is provided on the first transparent region 12. Hence, thefirst transparent region 12 allows passage of the light.

[0062] As shown in FIG. 5C, the chromium-less phase shift mask 2 has asecond transparent region 13 and a third transparent region 14. In thesecond transparent region 13, a phase shifter 20 is formed on thetransparent substrate 24 as is illustrated in FIG. 5D. In the thirdtransparent region 14, no phase shifters are provided on the transparentsubstrate 24. The phase shifter 20 shifts the phase of the beam passingthrough the second transparent region 13, with respect to the phase ofthe beam passing through the third transparent region 14. The phaseshifter 20 has such a thickness d that the beam output from it differsin phase by 180° from the beam input to it. The thickness d is definedas follows:

d=λ/{2×(n−1)}.  (3)

[0063] The second transparent region 13 has a phase shifter edge 16. Thephase shifter edge 16 is a boundary between the second transparentregion 13 and the third transparent region 14.

[0064] The region near the phase shifter edge 16 functions as a shieldsection. The beams passing through the second transparent region 13 andthird transparent region 14, respectively, are out of phase with respectto each other. The intensity of light is therefore almost nil atposition on a resist layer, where the image of the phase shift edge 16is formed. The chromium-less phase shift mask 2 substantially has ashield region, though no shield films are provided on it.

[0065]FIG. 6A shows a layout of patterns obtained when the chromium mask1 and the chromium-less phase shift mask 2 are laid one upon the other.The phase shifter edge 16 has a part 15 aligning with the centerline 10of the shield region 11 of the chromium mask 1. When the masks 1 and 2are laid one upon the other as shown in FIG. 6A, the centerline 10 ofthe shield region 11 overlaps the part 15 of the phase shifter edge 16of the second transparent region 13 shown in FIG. 5B. In FIGS. 5A and6A, the part 15 of the phase shifter edge 16 is indicated in thick line.The thick line does not mean that the boundary line is thick in part.

[0066] When exposure is carried out using the chromium-less phase shiftmask 2, the part of the resist layer which is near the part on which thephase shifter edge 16 is projected will not be exposed to light. Thatis, a resist pattern may be formed near the part on which the phaseshifter edge 16 is projected. However, no resist patterns will be formedin that part of the resist layer which is exposed to light through thechromium mask 1 in the first exposure step. In the exposure method ofthis invention, a resist pattern is formed in only a part of the resistlayer which is near the position where the phase shifter edge 16 of thesecond transparent region 12 is projected, and is also covered with theshield region 11.

[0067] As shown in FIGS. 6A to 6C, the width of the second transparentregion 13, that is, the distance s between the phase shifter edges 16,is greater than the width w of the shield region 11 of the chromium mask1. The distance s is one between the opposing edges of the secondtransparent region 13 that defines the phase shifter edges 16. The factthat the distance s is greater than the width w helps to enhance theresolution of the image focused on the resist, as will be describedlater.

[0068] The exposure method according to the present embodiment will nowbe described in detail. At first, the first exposure step is carriedout, using the chromium mask 1 shown in FIG. 5A. FIG. 12 illustrates areducing projection exposure apparatus 50 that is used to perform theexposure method. The apparatus 50 effects the first exposure step. Inthe first exposure step, the chromium mask 1 is secured to the maskholder 51 of the reducing projection exposure apparatus 50. The KrFexcimer laser 52 provided in the apparatus 50 emits a laser beam 53. Thewavelength of the laser beam 53 is 248 nm. The laser beam 53 is appliedat right angles to the chromium mask 1. As shown in FIG. 6B, the shieldfilm 21 reflects the laser beam 53 thus applied. The laser beam 53cannot pass through the shield region 11 and passes through the firsttransparent region 12 only. The laser beam 54 emerging from the firsttransparent region 12 passes through the projection lens 55 of thereducing projection exposure apparatus. The laser beam 54 is applied tothe positive-type resist 56 provided on a semiconductor substrate 57.

[0069]FIG. 6D shows the intensity distribution of the laser beam 54 atthe position on the positive-type resist 56, where the line C-C′ isprojected as shown in FIG. 6A. In FIG. 6D, the origin, at which x=0,coincides with the position where the centerline 10 of the shield region11 is projected at the surface of the positive-type resist 56. That is,the origin coincides with the position where the part 15 of the phaseshifter edge 16 is projected at the surface of the positive-type resist56. On the surface of the positive-type resist 56, the width w of theshield film 21 has such a value that a resist pattern may be formed,which is 120 nm (0.12 μm) wide along the x axis, i.e., the direction inwhich the image of the C-C′ line extends. The NA value, that is, thenumerical aperture of the projection lens 55 is 0.68.

[0070] The laser beam 54 applied to the to the positive-type resist 56in the first exposure step has the intensity distribution illustrated inFIG. 6D. As shown in FIG. 6D, the beam 54 is least intense at a positionwhere x=0. That is, the first dark section 3 is formed near thatposition where x=0. The broken line 40 in FIG. 6D represents the lowerlimit to the intensity of light that the positive-type resist 56responds to. The resist 56 is dissolved at any part that has beenirradiated with a laser beam having intensity higher than the intensityrepresented by the broken line 40. The intensity corresponding to thebroken line 40 varies with the conditions of the exposure process.

[0071] Then, the second exposure step is performed, using thechromium-less phase shift mask 2. As shown in FIG. 12, the chromium mask1 is removed from the mask holder 51 and the chromium-less phase shiftmask 2 is secured to the mask holder 51. A laser beam 53 is applied tothe chromium-less phase shift mask 2, at right angles as in the firstexposure step. The laser beam 53 passes through both the secondtransparent region 13 and the third transparent region 14 as isillustrated in FIG. 6C. As described above, the phase shifter 20 has thethickness d defined by the equation (3). Hence, the phase shifter 20imparts a phase difference of 180° to the two laser beams emerging fromthe second transparent region 13 and the third transparent region 14.The laser beam 58 which passed through the second transparent region 13and the third transparent region 14 passes through the projection lens55 and is applied to the positive-type resist 56.

[0072] The laser beams 58 applied to the resist 56 in the secondexposure step have the intensity distribution of FIG. 6E at the positionon the positive-type resist 56, where the line C-C′ is projected asshown in FIG. 6A. In FIG. 6E, the origin (x=0) is identical to theorigin of the graph (FIG. 6D) that represents the light-intensitydistribution observed in the first exposure step. The phase shifter 20has such a width that the image of the phase shift edge 16 is projectedon the positive-type resist 56, at a distance of 0.4 μm in the x axis.The incoherence ratio σ, or the incoherence of the beam emitted from theKrF excimer laser 52, is 0.3. It is desired that the incoherence ratio σbe as small as possible in the second exposure step.

[0073] In the second exposure step, the laser beam 58 applied to thepositive-type resist 56 has least intense at two positions, x=0 (μm) andx=0.4 (μm), as shown in FIG. 6E. That is, the beam is most weak at twopositions where the image of the phase shift edge 16 is projected on theresist 56. Two second dark sections 4 ₁ and 4 ₂ are formed,respectively, near that position where x=0 (μm) and near the positionwhere x=0.4 (μm).

[0074] The second transparent region 13 and the third transparent region14, through which two beams output of phase pass, sharply change thelight-intensity distribution at a position which is close to theposition where the image of the phase shift edge 16 is projected on thepositive-type resist 56. The intensities of the laser beams 58 muchchange at the boundary of the second dark sections 4 ₁ and 4 ₂. Thismeans that the widths of both dark sections 4 ₁ and 4 ₂ can bedecreased.

[0075]FIG. 6F represents half the total distribution of the light beamsapplied to the positive-type resist 56 in the first and second exposuresteps. As FIG. 6F shows, a third dark section 5 is formed on thepositive-type resist 56. The third dark section 5 is located near aposition that corresponds to the origin (x=0) of the graph (FIG. 6F).That is, the third dark section 5 is formed at a position where thefirst dark section 3 and one of the second dark sections 4 ₁ overlapeach other. Namely, the third dark section 5 is provided at the positionwhere a pattern is to be formed.

[0076] Also, no resist patterns should not be formed at the positionwhere the other second dark section 4 ₂ is provided. However, no resistpatterns will be formed at the position where the other second darksection 4 ₂ is provided. This is because a light beam having moreintense than is represented by the broken line 40 is applied at theposition where the other second dark section 4 ₂ is provided in thefirst exposure step.

[0077] Thus, the phase shift exposure technique according to the presentembodiment can be utilized to form two-dimensional random patterns.

[0078] In the second exposure step, the distance s between the phaseshifter edges 16 is desirably greater than the width w of the shieldregion 11 of the chromium mask 1. This results in that the phase shifteredges 16 of the phase shifter 20 reliably function as shield sections.The shorter the distance s between the edges 16, the shorter thedistance between the two dark sections formed in the second exposurestep. If the distance s is too short, no dark sections will be formed onthe positive-type resist 56. Hence, it is required that the distance sbetween the phase shifter edges be sufficiently long. The distance sgreater than the width w of the shield region 11 enhances the resolutionof the image focused on the positive-type resist 56. In addition,two-dimensional random patterns can be formed in high dimensionalprecision on the positive-type resist 56.

[0079] The exposure method according to the present embodiment can formimages on the positive-type resist 56 at higher resolution than ispossible with the conventional exposure method. FIG. 7 is a magnifiedrepresentation of that part of FIG. 6F which shows the light-intensitydistribution (solid line) at the third dark section 5, and illustratesthe light-intensity distribution (broken lines) observed in theconventional exposure method using a chromium mask 1 only. The exposuremethod of the invention increases the contrast about twice the valueachieved by the conventional exposure method. The word “contrast” usedhere is concerned with the light applied to the positive-type resist. Itmeans the ratio of the most intense part of the beam to the leastintense part thereof in terms of brightness.

[0080] Moreover, the exposure method of the invention can increase thedepth of focus more than is possible with the conventional exposuremethod described above. FIGS. 8A to 8F are diagrams showing thedefocus-dependency of light-intensity distribution, which is observed inthe exposure method according to the invention. FIGS. 9A to 9F arediagrams illustrating the defocus-dependency of light-intensitydistribution, which is observed in the conventional exposure method.“Defocus” here represents a difference in vertical directions from thesurface of the resist whose focal position is the image field. As seenfrom FIGS. 8A to 8F and FIGS. 9A to 9F, the depth of focus achieved inthe method of the invention is greater than that obtained in theconventional method. Thanks to the great depth of focus, two-dimensionalrandom patterns can be formed on the resist in the exposure methodaccording to the present invention.

[0081] In the exposure method described above, more light is desirablyapplied to the resist in the second exposure step than in the firstexposure step. The light-intensity distribution in the second exposurestep using the phase shift mask is sharper than the light-intensitydistribution in the first exposure step using no phase shift masks. Itis desired that light be applied to a part (x=0) of the resist, where aresist pattern will be formed, mainly in the second exposure step. Whenmore light is applied to the resist in the second exposure step than inthe first exposure step, the ratio of light applied in the secondexposure step to the light applied in the first exposure is greater.This further enhances the resolution of the image focused at the surfaceof the resist and ultimately increases the dimensional precision of thetwo-dimensional random patterns formed on the resist. In the firstexposure step, on the other hand, it suffices to apply a smaller amountof light to the second dark section 4 ₂ of the resist. This is becauseno pattern needs be formed on the second dark section 4 ₂.

[0082] The phase shifter edge 16 provided at the second transparentregion 13 of the chromium-less phase shift mask 2 can achieve the objectof the present invention, only if the overlapping parts of the chromiummask 1 and the phase shift mask 2 lie over the shield region 11. Aresist pattern is formed on only that part of the resist which is nearthe image of the phase shift edge 16 of the second transparent region 12projected on the resist and which is protected by the shield region 11.It is, however, desired that all centerline 10 of the shield region 11should align with the phase shifter edge 16 provided at the secondtransparent region 13. If the centerline 10 aligns with the phaseshifter edge 16, the least intense point in the distribution of thelaser beam applied in the first exposure step coincides with the leastintense point in the light distribution of the laser beam applied in thesecond exposure step. This enhances the resolution of any pattern formedon the resist.

[0083] The chromium mask 1 and chromium-less phase shift mask 2, shownin FIG. 5A and FIG. 5B, respectively, are nothing more than examples.The chromium mask and chromium-less mask that are shown in FIG. 10 mayreplace them. FIG. 10 is a plan view depicting the alternative chromiummask and chromium-less phase shift mask, which overlap each other. Asshown in FIG. 10, the distance s3 between the phase shifter edges of thechromium-less phase shift mask is equal to the distance between shieldregions. The term “distance between shield regions” means the distancebetween the centerlines of any two opposing shield regions. Like thechromium-less phase shift mask 2 described above, the chromium-lessphase shift mask shown in FIG. 10 has phase shifter edges spaced apartby distances (s1, s2, s4, s5) which are longer than the width of theshield region 11 of the chromium mask.

[0084] Also, the two beams that have passed through the second and thirdtransparent regions 13 and 14, respectively, are allowed to have a phasedifference that deviates a little from the desired value of 180°. Theexperiments the inventor hereof conducted reveal that thelight-intensity distributions of the beams applied to the resist in thefirst and second exposure steps have a desirable contrast so long as thephase difference falls within the range of 175° to 185°.

[0085]FIG. 11 shows a chromium-less phase shift mask 2′ designed forprinting patterns on substrates. The mask 2′ may be used in place of thechromium-less phase shift film 2. The chromium-less phase shift mask 2′has a transparent substrate 24′. The transparent substrate 24′ has aphase shifter 20′ that has been etched to a depth d′. The depth d′ isgiven as follows:

d′=/{2×(n′−1)},  (3′)

[0086] where n′ is the refractive index of the transparent substrate24′.

[0087] In the exposure method described above, it is possible to use thechromium-less phase shift mask in the first exposure step and thechromium mask in the second exposure step.

[0088] As has been described above, the exposure method according to thepresent invention comprises the first exposure step and the secondexposure step. A photo-mask having a two-dimensional random pattern isused in the first exposure step. A phase shift mask having a phase shiftedge pattern is used in the second exposure step. With the exposuremethod of the invention, it is possible to form two-dimensional randompatterns by the use of phase shift exposure technique. The exposuremethod of the invention can therefore enhance the resolution of imagesfocused on a resist. Moreover, the exposure method of the invention canform, on resists, two-dimensional random patterns of high dimensionalprecision.

[0089] Although the invention has been described in its preferred formwith a certain degree of particularity, it is understood that thepresent disclosure of the preferred form has been changed in the detailsof construction and the combination and arrangement of parts may beresorted to without departing from the spirit and scope of the inventionas hereinafter claimed.

What is claimed is:
 1. A method of forming a photoresist pattern by aphotolithography technique comprising: providing a photoresist layer;exposing said photoresist layer to a first pattern-defining light usinga first mask including a shielding region shielding said firstpattern-defining light; and exposing said photoresist layer to a secondpattern-defining light using a second mask including: a phase-shiftingregion having a phase shifter edge, wherein a first light portion ofsaid second pattern-defining light passes through said phase-shiftingregion, and a non-phase-shifting region adjacent to said phase-shiftingregion on said phase shifter edge, wherein a second light portion ofsaid second pattern-defining light passes through saidnon-phase-shifting region, and a first phase of said first light portiondiffers from a second phase of said second light portion, and whereinsaid first and second masks are aligned such that said phase shifteredge overlaps on said shielding region.
 2. A method according to claim 1, wherein said shield region comprises a line resist shielding portionto form a line resist pattern extending to a first direction, and saidline shielding portion has a centerline extending to said direction, andwherein said phase shifter edge substantially overlaps on saidcenterline when said first and second mask are aligned.
 3. A methodaccording to claim 2 , wherein said phase shifter edge comprises firstand second phase shifter edges parallel to each other and extending tosaid first direction, and wherein a distance between said first andsecond phase shifter edges is larger than a width of said line shieldingportion in a second direction perpendicular to said first direction. 4.A method according to claim 1 , wherein said phase-shifting region isprovided with a phase-shifter layer, and a thickness of saidphase-shifter layer is determined such that a phase difference betweensaid first phase and said second phase ranges from 175 to 185°.
 5. Amethod according to claim 1 , wherein said phase shifter edge comprisesfirst and second phase shifter edges parallel to each other andextending to a first direction, wherein said shield region comprises aline shielding portion for forming a line resist pattern, and extendingto said first direction, and wherein a distance between said first andsecond phase shifter edges is larger than a width of said line shieldingportion in a second direction perpendicular to said first direction. 6.A method according to claim 1 , wherein said first pattern-defininglight has a first intensity and said second pattern-defining light has asecond intensity, and wherein said second intensity is larger than saidfirst intensity.
 7. A mask set comprising: a first mask including: ashielding region shielding a pattern-defining light exposed to saidfirst mask; and a second mask including: a phase-shifting region havinga phase shifter edge, a non-phase-shifting region adjacent to saidphase-shifting region in said phase shifter edge, wherein a first phaseof a first light passing through said phase-shifting region differs froma second phase of a second light passing through said non-phase-shiftingregion, and wherein said phase shifter edge overlaps on said shieldregion when said first and second masks are aligned.
 8. A mask setaccording to claim 7 , wherein said shield region comprises a lineshielding portion to form a line resist pattern extending to a firstdirection, and said line shielding portion has a centerline extending tosaid direction, and wherein said phase shifter edge substantiallyoverlaps on said centerline when said first and second mask are aligned.9. A mask set according to claim 8 , wherein said phase shifter edgecomprises first and second phase shifter edges parallel to each otherand extending to said first direction, and wherein a distance betweensaid first and second phase shifter edges is larger than a width of saidline shielding portion in a second direction perpendicular to said firstdirection
 10. A mask set according to claim 7 , wherein said phaseshifter edge comprises first and second phase shifter edges parallel toeach other and extending to a first direction, wherein said shieldregion comprises a line shielding portion for forming a line resistpattern, and extending to said first direction, and wherein a distancebetween said first and second phase shifter edges is larger than a widthof said line shielding portion in a second direction perpendicular tosaid first direction
 11. A mask set according to claim 7 , wherein saidphase-shifting region is provided with a phase-shifter layer, and athickness of said phase-shifter layer is determined such that a phasedifference between said first phase and said second phase ranges from175 to 185°.